In cpu_fork() do not copy the register windows stashed in the u_pcb

In cpu_fork() do not copy the register windows stashed in the u_pcb
if the new proc has its own stack. In that case the new thread starts
with a fresh stack and there are no register windows to spill back.

This fixes the pthread issues that kurt@ was struggling with for a long
time. Depending on the scheduler a thread would stomp over another threads
stack which caused strange crashes.

Improved diff provided by miod@
OK miod@ mpi@ kurt@